A wide variety of MEMS devices are known, including accelerometers, rate sensors, actuators, motors, microfluidic mixing devices, springs for optical-moving mirrors, etc. As an example, rotational accelerometers that employ MEMS devices are widely used in computer disk drive read/write heads to compensate for the effects of vibration and shock. Other applications for rotational accelerometers that use MEMS devices include VCR cameras and aerospace and automotive safety control systems and navigational systems. Rotational rate sensors and accelerometers have been developed whose MEMS devices are fabricated in a semiconductor chip. Notable MEMS devices that employ a proof mass for sensing rotational rate or acceleration include a plated metal sensing ring disclosed in U.S. Pat. No. 5,450,751 to Putty et al., and an electrically-conductive, micromachined silicon sensing ring disclosed in U.S. Pat. No. 5,547,093 to Sparks, both of which are assigned to the assignee of this invention. Sparks' sensing ring is formed by etching a chip formed of a single-crystal silicon wafer or a polysilicon film on a silicon or glass handle wafer. A sensor disclosed in U.S. Pat. No. 5,872,313 to Zarabadi et al., also assigned to the assignee of the present invention, employs a sensing ring and electrodes with interdigitized members. The positions of the interdigitized members relative to each other enable at least partial cancellation of the effect of differential thermal expansion of the ring and electrodes, reducing the sensitivity to temperature variations in the operating environment of the sensor. Each of the above sensors operates on the basis of capacitively sensing movement of their rings. The sensing rings are supported by a central hub or pedestal. Surrounding the rings are drive electrodes that drive the rinses into resonance, while sensing electrodes that also surround the rings serve to capacitively sense the proximity of the ring (or nodes on the ring), which varies due to Coriolis forces that occur when the resonating rinse is subjected to rotary motion.
Another notable MEMS device that employs a silicon proof mass for sensing rotational acceleration is disclosed in U.S. patent application Ser. No. 09/410,712 to Rich, incorporated herein by reference. Rich discloses a disk-shaped proof mass supported above a cavity formed in a substrate. Instead of being centrally supported by a pedestal, Rich's proof mass to is suspended from its perimeter with tethers anchored to the substrate rim surrounding the proof mass. The tethers allow the proof mass to rotate about an axis perpendicular to the plane containing the proof mass and tethers. Fingers extend radially outward from the proof mass and are interdigitized with fingers extending radially inward from the substrate rim. The cantilevered fingers of the proof mass and rim are capacitively coupled to produce an output signal that varies as a function of the distances between adjacent paired fingers, which in turn varies with the angular position of the proof mass as it rotates about its axis of rotation in response to a rotational acceleration.
Sensors of the type described above are capable of extremely precise measurements, and are therefore desirable for use in a wide variety of applications. However, the intricate proof masses and associated sensing structures required for such sensors must be precisely formed in order to ensure the proper operation of the sensor. For example, Rich's device requires a sufficient gap between paired interdigitized fingers to prevent stiction and shorting, yet paired fingers must also be sufficiently close to maximize the capacitive output signal of the sensor. Rich employs stiction bumps formed on the proof mass fingers to inhibit stiction between closely-spaced fingers. Increasing the area of the fingers to achieve greater capacitive coupling would result in increased capacitive output for a given finger gap. However, traditional etching techniques have not generally been well suited for mass-producing silicon micromachines with high aspect ratios necessary to etch closely-spaced fingers in a relatively thick substrate. For example, with conventional etching techniques it is difficult to achieve a 10:1 aspect ratio capable of forming interdigitized fingers spaced three micrometers apart in a silicon substrate that is thirty micrometers thick. In addition to operational considerations, there is a continuing emphasis for motion sensors that are lower in cost, which is strongly impacted by process yield, yet exhibit high reliability and performance capability. Consequently, improvements in the processing of MEMS devices for sensing and other applications are highly desirable. Deep reactive ion etching (DRIE) is a process known as beings capable of performing deep, high aspect ratio anisotropic etches of silicon and polysilicon, and is therefore desirable for producing all-silicon MEMS of the type taught by Rich. However, DRIE is a young technology practiced largely for research and development. Accordingly, to take advantage of the unique capabilities of the DRIE process, its etch idiosyncrasies must be determined and reconciled to render it suitable for mass production.